Recently, importance has been placed on the cleanliness of exhaust gas and reduction in the discharge of CO2 with respect to automobile engines and the like. There is a limit in meeting these requirements with the combustion of fossil liquid fuel in conventional engines. There have been commercialized or developed gas combustion engines that burn such fossil fuel as gaseous fuel derived from natural gas, petroleum gas, and moreover hydrogen gas.
With conventional gaseous fuels, sufficient durability cannot be ensured in in-cylinder direct injection engines because of their poor lubricity. Specifically, various problems, including wear and seizure in sliding parts in injectors and nozzles and poor sealability for fuel due to wear in valve seats, have arisen. In compression ignition engines, such fuels as hydrogen and compressed natural gas low in cetane number are inferior in ignitability, and they make operation unstable.
Methods for solving the above-mentioned problems are disclosed in, for example, JP-2003-232234A, JP-S63-4365U, and JP-H1-88054U.
In the method disclosed in JP-2003-232234A, the injection of gaseous fuel and liquid fuel is controlled according to engine operating conditions, and the running performance is thereby enhanced to increase output and reduce exhaust gas. However, this method has a drawback. The engine used in the method has a wide range of operation by liquid fuel for an engine that uses gaseous fuel as main fuel, and it is required to provide two large fuel tanks. Further, a fuel injection valve and a supply circuit are respectively required for two systems, one for gas and one for liquid.
In JP-S63-4365U, there is disclosed such a construction that aside from working fluid for driving an injector, mineral seal oil is introduced into an injector to ensure lubricity for the sliding parts of a needle. However, this method also has drawbacks. The construction is complicated, and mineral seal oil pressure is constantly applied to a needle seal groove, which causes the mineral seal oil to constantly leak into working fluid and gaseous fuel.
The method disclosed in JP-H1-88054U is simpler in construction than the method disclosed in JP-S63-4365U in that working fluid is also used as lubricating oil. In the method, working fluid and gaseous fuel collect in a mineral seal oil sump, and the lubricity for a needle is thereby ensured. However, this method also involves a problem. It is difficult for working fluid to get into the lower part of the mineral seal oil sump, e.g., the nozzle hole side of the sump, because of gaseous fuel pressure; therefore, the lubricity for the lower part of a needle cannot be ensured.
In both the method disclosed in JP-S63-4365U and the method disclosed in JP-H1-88054U, the lubricity is ensured only for sliding parts. However, these methods have drawbacks that wear in a valve seat cannot be reduced and fine liquid fuel injection for ensuring ignitability cannot be carried out.
Gaseous fuel is prone to leak; namely, a leak may occur from an injector having a portion, where metallic surfaces are in tight contact, and get out of a fuel circuit. These methods are not provided with a construction or a function for coping with leaking gaseous fuel.